Input and output terminal units for data processing systems have commonly provided magnetic stripe credit card readers for inputting customer account data into the system, such readers having been characterized by a variety of means and structures for either transversely activating a read head relative to a stationarily positioned credit card, or for transversely activating the credit card and the magnetic stripe thereof relative to a stationarily positioned read head. Regardless of whether the card is transversely activated relative to the read head, or the head relative to the magnetic stripe, these credit card readers have most generally provided cumbersome and relatively expensive means for controlling the velocity of the moving member's transverse motion, the latter being a requirement for the accurate and reliable reading and inputting of the recorded data. Although known structures for transversely activating the read head relative to the card have tended to be less expensive to manufacture than known structures for activating the credit card and magnetic stripe relative to the head, these structures have generally required the use of a servo motor in association with control circuitry to assure a constant and uniform velocity of movement of the read head relative to the magnetic stripe. Early attempts at a further reduction in the manufacturing costs of magnetic stripe readers, by eliminating the servo motor and the associated control circuitry, involved the employment of various spring drives as a means of transversely activating the head relative to the stripe. In these earlier spring powered readers, the read head carrier would be manually activated in a first direction to expand a drive spring and to set the head in its read-start position, whereupon the drive spring would activate the carrier in a second direction as the read head traverses the magnetic stripe and readably inputs the data recorded thereon. These spring drives however, by reason of the reducing pulling force of their contracting springs, were found to produce undesirable fluctuations in the velocity of travel of the read head relative to the magnetic stripe, the required constant and uniform velocity of travel of the read head accordingly not having been achieved thereby.